1. Field of the Invention
The present invention relates to electronic power control devices and specifically to MOS-gated devices, such as MOSFET transistors and insulated gate bipolar transistors, that switch high voltages to motors and other loads in half-bridge and full-bridge multiphase configurations.
2. Description of the Prior Art
Digital logic is an almost ideal mechanism for switching power on and off to loads, especially with high-current, high-voltage MOSFET transistors that do the actual switching. However, many loads such as motors and fluorescent lights operate at voltages substantially higher than the five volts direct current (DC) used by most digital logic. Some form of level shifting is required to interface the MOSFET switch that floats at high-voltage and the digital logic that is ground referenced.
FIG. 1 illustrates a half-bridge circuit 10 for driving a fluorescent light 12. A pair of MOSFET transistors 14 and 16 switch their junction between ground and a positive high-voltage potential (+HV). The lower end of light 12 is thus driven between ground and +HV through an impedance matching circuit comprising a capacitor 17 and an inductor 18 An oscillator 19 is connected to a low-side driver 20 which in turn controls the gate of transistor 16 and a high-side driver 22 that controls the gate of transistor 14. When transistor 14 is on, transistor 16 will be off, and vice versa. A logic supply voltage (Vdd) powers low-side driver 20 directly and high-side driver 22 indirectly with the help of a bootstrap diode 24 and a bootstrap capacitor 26. Low-side driver 20 controls high-side driver 22 through a pair of control lines 28 and 30. A comparator in high-side driver 22 will latch the switch transistor 14 on and off according to the relative voltages between lines 28 and 30.
FIG. 2 illustrates the basic functional parts of low-side driver 20 which comprises a inear regulator 40, an inverter 42 with hysteresis, an under-voltage lockout 44, a buffer with hysteresis 46, a NAND-gate 48, a pulse circuit 50, a pair of transistors 52 and 54, and an output inverter driver 56 which controls the gate of transistor 16. An input 58 receives a control signal that is combined in NAND-gate 48 with an under-voltage signal from lockout 44. If the logic power Vdd falls below a predetermined threshold, driver 56 will be prevented from turning transistor 16 on. An input 60 receives an inverted control signal that will turn transistor 14 on and off by pulsing either transistor 52 or transistor 54 on and the other being off. Which line, 28 or 30, is lower than the other will be sensed by high-side driver 22 and used to ultimately control the gate of transistor 14.
FIG. 3 illustrates high-side driver 22 which comprises a linear regulator 70, a discriminator 72, an under-voltage lockout 74, an AND-gate 76, a flip-flop 78, an output driver 80, a pair of constant current sources 82 and 84, a pair of transistors 86 and 88, a pair of Zener diodes 90 and 92, and a pair of input pull-up resistors 94 and 96. An output 98 is connected to driver 80 and the gate of transistor 14. A reference common 99 connects to the source of transistor 14 and can swing from below ground in circuit 10 to above +HV, in some cases. Diode 24 takes advantage of these swings to provide a supply voltage Vddh. If the supply voltage Vddh falls below a predetermined threshold, driver 80 will be prevented from turning transistor 14 on. Transistors 86 and 88 in a common source configuration present a high-impedance comparator input to control lines 28 and 30. If the voltage on line 30 is more negative than a predetermined threshold, e.g. Vddh-1.5 volts, and transistor 52 (FIG. 2) is off and transistor 54 is on, then the set (S) input of flip-flop 78 will go true, turning transistor 14 (FIG. 1) off. If the voltage on line 28 is more negative than the predetermined threshold, and transistor 54 (FIG. 2) is off and transistor 52 is on, then the reset (R) input of flip-flop 78 will go true, turning transistor 14 on. Table I summarizes the control function. Symbols S, R and Q are the set, reset and output, respectively, of flip-flop 78. The asterisk in the first two columns indicates negative true logic, HIGH=more negative input than threshold voltage, LOW=less negative than threshold voltage.
TABLE I ______________________________________ Line 28* Line 30* S R Q ______________________________________ HIGH LOW HIGH LOW HIGH LOW HIGH LOW HIGH LOW HIGH HIGH LOW LOW No Change LOW LOW LOW LOW No Change ______________________________________
A problem develops in discriminator 72 with 5 such a simple input and logic. Noise on lines 28 and 30 can easily be induced and proper circuit 10 operation depends on a high degree of common mode noise rejection. If an imbalance develops between lines 28 and 30, such as can happen with unequal stray line capacitances and high frequency environments, false triggering of transistor 14 can occur. These false triggers can be very serious if they occur while transistor 16 is on, because +HV will momentarily find a short path to ground through transistor 14, causing a high current pulse.
A high-side driver is needed that is immune to false triggering caused by rapid common mode slewing of the control lines. An improved threshold region of the comparators is needed to prevent a slight mismatching of the control lines to trigger an unintended change of state.